1. Field of the Invention
The present invention relates to an apparatus and method for amplifying a signal, and a wireless transmitter using the same.
2. Description of Related Art
Lately, a mobile communication system has been advanced to transmit data at a high data transmission rate. Accordingly, a related system has been advanced from employing a Code Division Multiple Access (CDMA) scheme to employing an orthogonal frequency division multiplexing (OFDM) scheme. For example, the OFDM scheme has been adapted to a Worldwide Interoperability for Microwave Access (WiMAX) system, a Wireless Broadband (WiBro) system, and a 3rd Generation Long-Term Evolution (3G LTE) system.
However, such an OFDM system is disadvantageous in that a peak to average power ratio (PAPR) of a transmission signal increases by summation of a subcarrier. Therefore, many researches have made effort to improve the efficiency of a transmitter of a terminal. As a related method for improving the efficiency, a phase signal is inputted to the power amplifier using polar coordinate conversion and envelope information is applied to a bias unit of a switching power amplifier. The envelope information may be inputted as an analog signal to the bias unit without conversion, or the envelope information may be inputted as a digital signal to the bias unit through analog-digital conversion.
FIGS. 1A and 1B are diagrams illustrating a transmitter according to the prior art.
The transmitter according to the prior art, shown in FIG. 1A, applies envelope information as an analog signal to the bias unit without conversion. Referring to FIG. 1A, the transmitter according to the prior art includes a MODEM 101, a polar coordinate converter 102, an analog convertor 103, a phase modulator 104, a switching power amplifier 105, and a power source 106. The MODEM 101 outputs baseband signals I(t) and Q(t). The polar coordinate converter 102 receives the baseband signals I(t) and Q(t), converts the received baseband signals to a polar coordinate, and outputs phase information and envelope information. The phase modulator 104 up-converts phase information to a radio frequency (RF) and outputs a predetermined sized envelope signal to the switching power amplifier 105. Meanwhile, the envelope information outputted from the polar coordinate converter 102 is applied to the bias unit of the switching power amplifier 105 after passing through the analog converter 103. The switching power amplifier 105 combines the phase information and the envelope information applied to the bias unit and outputs the combined signal. The power source 106 supplies power to the switching power amplifier 105.
A class-B amplifier or a class-AB amplifier may be used as the analog converter 103. However, the transmitter of FIG. 1A is not suitable to a system having an abruptly changing envelope signal because the envelope information is applied to the power amplifier 105 as bias. For example, the OFDM system cannot express an envelope signal smaller than a knee voltage because a VDD/VCC voltage should be greater than a knee voltage to always activate the switching power amplifier 105.
FIG. 1B shows a transmitter applying envelope information as a digital signal to the bias unit. Referring to FIG. 1B, the transmitter includes a MODEM 111, a polar coordinate converter 112, a digital converter 113, a phase modulator 114, a switching power amplifier 115, a power source 116, a power controller 117, and a band pass filter 118. The polar coordinate converter 112 receives baseband signals I(t) and Q(t) from the MODEM 111, converts the received baseband signals I(t) and Q(t), and outputs phase information and envelope information. The phase information is inputted to the switching power amplifier 115 through the phase modulator 114 like the transmitter of FIG. 1A. The digital converter 113 converts the envelope information outputted from the polar coordinate converter 112 to a digital signal in pulse having a predetermined bit sequence. The digital converter 113 may be embodied as a delta-sigma converter. The switching power amplifier 115 receives the pulse type envelope information as bias and combines the received pulse type envelope information with the phase information, and outputs the combined signal. The power source 116 supplies power to the switching power amplifier 115 and the power controller 117 controls the power to the switching power amplifier 115.
The transmitter of FIG. 1B essentially generates quantization noise due to conversion of bit sequence of envelope information. In order to remove quantization noise, the transmitter includes a band pass filter for filtering the output of the switching power amplifier 115. When the transmitter uses the delta-sigma convertor as the digital converter 113, noise shaping of quantization noise is decided by the oversampling rate of envelope information and the order of the delta-sigma converter. In general, a transmitter uses a 2nd order delta-sigma converter for system stability. In this case, the oversampling rate of envelope information should be sustained at 16 to 32 in order to remove in-band and out-band noise by a filter. Lately, a high speed data transmitter has wideband characteristics such as a channel bandwidth from 20 MHz to 80 MHz. Therefore, a delta-sigma converter needs to perform oversampling at a high speed such as 2.56 GHz in case of 32 oversampling. Due to such a requirement, it is very difficult to embody the system in hardware. Also, power consumption increases due to a high speed digital circuit.